The nuclear quadrupole moment is a measure of the deviation of the nuclear charge distribution from spherical symmetry. There is a quantized electrostatic interaction energy between the nuclear quadrupole moment and the electric field gradient tensor resulting from the surrounding electronic charge. This leads to preferred orientations of the nucleus with quantized energy levels.
A quadrupolar nucleus has a magnetic moment that is related to the intrinsic angular momentum, or spin, of the nucleus. The magnetic moment of the nucleus is also quantized, and for nitrogen-14 it can take on three values.
The orientation of the nucleus can be perturbed by applying an oscillating magnetic field whose frequency corresponds to the energy difference between any two preferred nuclear orientations. The energy difference, and associated transition frequency, between any two levels is not necessarily sharp. Impurities, mechanical strains, and temperature gradients in a material all lead to a distribution of resonant frequencies, or lineshape.
In conventional NQR detectors, excitation of a QR response requires the application of a pulsed RF magnetic field within the search volume. The applied RF pulse may excite spurious responses from materials within the search volume that can obscure the QR response, leading to an unacceptably large false alarm rate. Examples of internal noise sources include the decaying magnetic field generated by currents induced within conductive materials located within the search volume, as well as piezoelectric responses from materials within the search volume. In order to excite a QR response, the amplitude of the RF field is typically larger than several Gauss over the search region. This level of amplitude requires the use of a costly power amplifier and excitation probe, and exceeds the allowable limits for human exposure.